Improve error reporting in code that checks for buffer refcount leaks.

Formerly we just Assert'ed that each refcount was zero, which was quick
and easy but failed to provide a good overview of what was wrong.
Change the code so that we'll call PrintBufferLeakWarning() for each
buffer with a nonzero refcount, and then Assert at the end of the loop.
This costs nothing in runtime and might ease diagnosis of some bugs.

Greg Smith, reviewed by Satoshi Nagayasu, further tweaked by me
Improve the documentation about commit_delay.

Clarify the docs explaining what commit_delay does, and add a
recommendation about a useful value for it, namely half of the single-page
fsync time reported by pg_test_fsync.  This is informed by testing of
the new-in-9.3 implementation of commit_delay; in prior versions it
was far harder to arrive at a useful setting.

In passing, do some wordsmithing and markup-fixing in the same general
area.

Also, change pg_test_fsync's default time-per-test from 2 seconds to 5.
The old value was about the minimum at which the results could be taken
seriously at all, and so seems a tad optimistic as a default.

Peter Geoghegan, reviewed by Noah Misch; some additional editing by me